1. Field of the Invention
This invention pertains to a method for thermally developing a photosensitive element, and particularly to a method that includes heating the element to cause a portion of a composition layer to liquefy, and providing a development medium comprising a support and an absorbent material in contact with the element to remove the liquefied material.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
U.S. Pat. No. 3,060,023 describes a dry thermal image reproduction process for transferring images from photopolymerized image-bearing elements to a receptor surface. The type of receptor support is dependent on the desired use for the transferred image and on the adhesion of the image to the base. Disclosed receptor supports include paper; cardboard; metal sheets, foils, and meshes; wood; glass; nylon; rubber; polyethylene; linear condensation polymers such as the polyesters, e.g., polyethylene terephthalate; silk, cotton, and rayon fabrics or screens. The receptor supports were used in sheet form. The process can be used to prepare relief images ranging in depth from a fraction of a mil up to 10 mils or more by thermal transfer of the unexposed areas of the photopolymerizable stratum. The purpose of the process is to reproduce the transferred image on the receptor to provide at least one duplicate copy of the original image. Example IV describes transfer of an image to a receptor support made of a fine mesh screen of silk mounted on a wooden frame and supported by a firm aluminum sheet.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 and U.S. Pat. No. 6,797,454 each describe an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element with a web of absorbent material. The apparatus includes a hot roll that delivers the absorbent material to the photosensitive element. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact at a nip so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy. The absorbent web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and then is separated from the element
A problem sometimes arises during thermal development where the absorbent material is a continuous web, and in particular, a web of nonwoven material. After the absorbent material contacts the is photosensitive element and collects the softened unirradiated material, the web of absorbent material can stretch and/or distort while being separated from the photosensitive element. The adhesion or the ability to separate the absorbent web from the element varies with the relief image forming. Portions of the relief image that are polymerized and therefore less tacky, peel easily as the web separates. Whereas the absorbent web may adhere and peel after the nip in portions of the relief image that are unpolymerized and thus are tacky or molten polymer. At times, the web adheres to the photosensitive element to such an extent that the can web stretch and/or distort while being separated or peeled from the element. Forces associated with the peeling of the web from the element change when the web stretches and/or distorts, which can induce defects into the element such as waves, variations in relief formation, etc. Printing with printing forms having variations in relief can be a problem particularly for high quality printing as areas with shallow relief can accumulate dirt that ultimately prints on the substrate, and relief areas that are too deep can weaken fine printing elements such as highlight dots and fine lines.
The stretching and/or distorting web can adhere to the photosensitive element to such an extent that the web can even cause the photosensitive element to lift from its support surface while being separated or peeled from the element. The removal of the absorbent web from the still warm photosensitive element can induce defects in the resulting relief element. Stretching and/or distorting of the web particularly while peeling, and the lifting of the photosensitive element while the element is still hot, can bend the element and induce strains in the structure of the element which create a defect, called waves, in the resulting relief element. The non-uniform strains imparted in the element while the support is at a temperature higher than the glass transition temperature result in deformations that remain after the element has cooled or returned to room temperature. The deformations are waves of localized distortions resulting in a non-planar topography of the photosensitive element. Because of the uncontrolled nature of the web in thermal development of the prior art, waves of distortions can form in different locations in each element processed.
Relief printing forms having waves result in poor print performance. In multicolor printing, when one or more of the relief printing forms have waves the printed image has poor registration. Even in single color printing, waves in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. Further, the relief printing form having waves may incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.
Further, the web may stretch and remain in contact with the photosensitive element for a sufficient time that the molten polymer starts to cool. Removal of the web while the element is cooling can change the cohesive and adhesive forces between the web and the relief surface of the photosensitive element. As such the relief surface may clean out, that is, remove unpolymerized polymer, differently than if the web is removed while the element is hot when directly in or immediately after the nip. Removal of the web while the element is cooling can lead to stripwise (across the web) artifacts in the relief surface such as dot size variations or variation in the quality and cleanliness of sidewalls of dots, that print as density variations.
Stretching or distortion of the continuous web during separation from the element can create other problems. Web tension changes as the web stretches. Controlling the velocity of the web can be more difficult since web velocity varies through the process due to the stretching of the web. The web may even break from extensive stretching or distortion. In this case, the web would not be present to remove the tacky molten polymer from the heated photosensitive element, and the polymer can flow onto various surfaces in the processor including the hot roll and the drum support roll. As such operations are suspended for a considerable downtime while the web is re-threaded through the processor and the tacky molten polymer is removed from various interior surfaces. If the molten polymer remains on the hot roll, the polymer tends to build up and harden on the roll, which can then impress patterns into the surface of subsequently processed printing forms.
It is thus desirable to provide a method for thermal development using a continuous web of the development medium that does not stretch, distort, or break. It is also desirable to provide a method for thermal development using a continuous web of the development medium that allows for relatively high tension in the web and more controlled separation of the web from the photosensitive element.